Core-shell particles having non-polar outer surface and methods for producing a three-dimensional object from the particles

ABSTRACT

The invention relates to particles for producing a three-dimensional object by using layer-building methods (powder-based generative rapid prototyping methods), to methods for producing a three-dimensional object therefrom, and to an object that can be produced by using the particles or the methods. The aim of the invention is to improve the precision of production methods of this type by preventing the tendency of the particles used to agglomerate. To this end, a surfactant layer is applied, whereby the nonpolar groups of the surfactants are oriented toward the particle surface thus forming a hydrophobic surface having a low tendency to agglomerate.

CROSS REFERENCE TO RELATED APPLICATION

This application is a national stage of PCT/DE 03/02013 filed 16 Jun.2003 and based upon DE 202 20 325.5 filed 18 Jun. 2002 and DE 203 08744.5 filed Mar. 26, 2003 under the International Convention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to particles for producing a three-dimensionalobject by means of layer-building processes (powder-based generativerapid prototyping processes), to processes for producing athree-dimensional object therefrom, and to an object producible usingthe particles or the processes.

2. Related Art of the Invention

Powder-based generative rapid prototyping processes are known, forexample, under the names 3D laser sintering or 3D printing.

The process known as 3D printing is a process for producing athree-dimensional object in which first of all a layer of particles isapplied to a target surface, and then selected parts of the layer,corresponding to a cross-section of the object, are printed with aliquid in which the particles are at least partially (superficially)soluble, in such a manner that the particles are joined to one anotherin the selected parts. Then, the application and printing steps arerepeated to build up a plurality of layers, so that the joined parts ofthe adjacent layers are joined together in order to form the object.

A 3D printing process of this type is known, for example, from EuropeanPatents EP 0 644 809 B1, EP 0 686 067 B1.

3D printing processes which join the particles by partially dissolvingthem with the binder liquid have the drawback that the finished objectis subject to significant shrinkage compared to the region of theparticle layer which was originally printed with the binder liquid. Thereason for this is that partially dissolved particles which are incontact with one another move closer together under the influence oftheir surface tension, so that a more tightly packed arrangement ispresent after drying of the binder liquid than before. This effectcannot readily be suppressed, and is indeed necessary to a certainextent in order to achieve sufficiently strong cohesion of the particlesin the finished object. However, one serious disadvantageous consequenceof this effect is that in the case of an object produced using a processof this type which exceeds a certain maximum size, the shrinkage duringthe drying process can lead to deformation or crack formation.

To combat this problem, 3D binder printing processes have been developedin which the binder liquid contains additives (sintering aids) whichremain behind in the printed regions of the layer after the liquid hasdried and make it possible to join the particles in the wetted regionsby the entire mass of particles which has been processed, including theregions which have not been printed, being heated in such a way that theparticles in the printed regions sinter together under the influence ofthe sintering aid, but the particles which have remained unprinted donot sinter together.

One problem of this technique is that the sintering aids used aregenerally of a mineral type and are at best dispersible but not solublein the binder liquid, which means that they cause considerable wear tothe spray nozzles used to wet the granulated material.

A further problem of the known 3D binder printing processes is that as aresult of agglomeration of the particles used, objects produced therebytend to have a non-uniform, rough surface profile which does notprecisely correspond to the profile of the printed regions.

The process known as 3D laser sintering is very similar to 3D printing.The only difference is that the printing with the binder liquid isreplaced by irradiating with an energy beam which superficially softensor melts the particles and thereby joins them. A process of this type isknown, for example, from DE 690 31 061 T2.

DE 103 13 452 A1, which was published after the priority date of thepresent application, has already proposed coated particles havingadvantageous properties for 3D laser sintering, the coating of whichparticles may also be hydrophilic. Particles of this type, as a resultof taking up water from the air, tend to agglomerate in an undesirableway, a phenomenon which should be avoided in order to ensure componentproduction which is as accurate as possible.

SUMMARY OF THE INVENTION

It is an object of the invention to provide particles for producing athree-dimensional object by means of layer-building processes whichavoid one or more of the drawbacks listed above, as well as processesfor producing a three-dimensional object therefrom and an objectproducible using the particles or the processes.

The object is achieved firstly by particles for producing athree-dimensional object by means of layer-building processes.

First of all, a polar intermediate layer, for example polymethacrylateor polyvinyl acetal, is applied to cores of any desired material, e.g.metal, ceramic or plastic, and then a surfactant layer is applied to thepolar intermediate layer. The surfactants are forced to arrangethemselves on the polar intermediate layer in such a manner that theirpolar groups face toward the intermediate layer and the nonpolar groupsface outward. Coated particles of this type have a hydrophobic surfaceand therefore do not take up any water from the environment. Thisresults in good flow properties and a significant reduction in thetendency to agglomerate.

Therefore, the particles according to the invention can be used toproduce objects with a smoother surface than when using a conventionalgranulated material, or objects with finer, more detailed structures canbe produced using the same particle size compared to conventionalparticles.

If the intermediate layer is relatively thick (0.1 to 10 percent of theparticle radius), a second benefit, which is independent of the polarityof the surface layer, is achieved. Specifically, on account of theintermediate layer and the material below it having differentchemical-physical properties, it is possible to restrict the partialfusion of the particles, which is required to produce a solid objectfrom the particles, to the intermediate layer and therefore, dependingon the ratio of the thickness of the intermediate layer to the materialbelow it, to limit the shrinkage. This applies both to 3D printing andto 3D laser sintering.

Thicknesses of the surface layer in the range from 0.1 to 10% of themean particle radius have proven suitable for this purpose.

Polyvinylpyrrolidones and acrylic polymers have proven particularlysuitable materials for an intermediate layer of this type.

The surface layer of the particle consists of surfactant. Surfactantsare generally characterized in that they combine polar and nonpolargroups in one molecule, so that they are able to make nonpolarsubstances soluble in polar solvents or vice versa on account of thefact that the polar group in each case accumulates at the polarsubstance and the nonpolar group accumulates at the nonpolar substance.In this case, the thickness of the surfactant layer corresponds asaccurately as possible to one monolayer, so that the polar groups of thesurfactant molecules are as far as possible all directed toward theinterior of the particles but the nonpolar groups of the surfactantmolecules are all directed outward and thereby form the uniformallynonpolar outer surface of the particle.

The surfactant may be any surfactant which is known from the field ofdetergents, cleansing agents or body-care agents. Anionic surfactants(e.g. fatty alcohol sulfates, alkylbenzene sulfates), cationicsurfactants (e.g. tetraalkylammonium salts), nonionic surfactants (e.g.fatty alcohol polyglycol ethers or alkyl polyglycosides) or amphotericsurfactants are suitable. By way of example, mention may be made ofsodium lauryl sulfate or betaine.

The surfactant and intermediate layer are expediently selected in such away that there is a solvent in which the surfactant is soluble but theintermediate layer is not. This means that it is possible for theintermediate layer to be applied to the particles first of all usingknown processes (e.g. spray drying) and then for the surfactant layer tobe applied on top of it by the particles provided with the intermediatelayer being brought into contact with a very dilute solution of thesurfactant and being dried through evaporation of the solvent. Thestrong dilution of the solution ensures that only a monomolecular layerof surfactant is formed on the surface of the intermediate layer.

In the embodiment explained above, the particles may have a core ofmetal, ceramic or polymer material. The core and coating materials, forthe particles to be used in a 3D printing process, should expediently beselected in such a way that a solvent exists which dissolves the surfacelayer and the intermediate layer but not the core. A solvent of thistype can be used as binder liquid in the subsequent 3D printing process.Although this binder liquid partially dissolves the layers surroundingthe core and thereby allows the layers of adjacent particles to fusetogether, since it does not attack the core itself, the shrinkage causedby the fusion is reduced to an extent which is at most proportional tothe ratio of the radius of the core to the thickness of the surfacelayer and if appropriate of the intermediate layer.

Correspondingly, if the particles are used for a 3D laser sinteringprocess, the core and coating materials should be selected in such a waythat the softening point of the core is well above that of the layersabove it, so that even with joining of this type the fusion of theparticles is restricted to the upper layers and therefore the shrinkageis minimized. It is particularly advantageous, although not necessary,to use a material with a low softening point, preferably <70° C., forthe production of the intermediate layer. The use of particles whichhave been coated in this way allows the process temperature to bereduced and therefore allows the shrinkage to be lowered further.

BRIEF DESCRIPTION OF THE DRAWING

Further features and advantages of the present invention will emergefrom the following description of exemplary embodiments and also fromFIG. 1:

FIG. 1: Shows particles which have been joined after printing orirradiation (in hatched form) surrounded by unjoined particles. Theintermediate layer and surfactant layer are illustrated in thickenedform to improve the clarity of the illustration.

DETAILED DESCRIPTION OF THE INVENTION

According to a first embodiment of the invention, a particle is in theform of a sphere. However, it will be understood that it may also take aform which deviates from the shape of a sphere, for example ellipsoidalor irregular form. The particle has a core of a polymer material, inthis case of polymethyl methacrylate (PMMA), which is surrounded by anintermediate layer formed from polyvinylpyrrolidone (PVP). This materialforms a polar, hydrophilic outer surface. Suitable polyvinylpyrrolidonesare marketed under the names Luviskol and Luvitec by BASF; an acrylicpolymer marketed under the name Bellac by Belland AG is also suitable.

The intermediate layer is produced by dissolving thepolyvinylpyrrolidone in an aqueous medium, applying the solution to theparticles and drying the particles. The solvent used in this case ispreferably water, since this has the advantage of evaporating withoutleaving any residues. (However, ethanol or an aqueous ethanol solutionis also suitable.) For the coating operation, the particles arefluidized in a fluidized bed by a hot airstream and are simultaneouslysprayed with the solution. Drops of the solution which come into contactwith particles and surround the latter evaporate in the hot airstream,with the result that the dissolved polyvinylpyrrolidone remains behindand forms the intermediate layer. The resulting layer thickness can becontrolled using the concentration of the solution/suspension/dispersionused and the duration of the treatment, as well as the temperature inthe fluidized bed.

Alternatively, it is also possible to apply an intermediate layer of theacrylic polymer Bellac by the latter being dissolved in a basic aqueousmedium with a pH of at least 10.

The surface layer formed from the surfactant sodium lauryl sulfate isproduced in a similar way to the intermediate layer of PVP by sprayingthe particles, which have been fluidized in the fluidized bed, with asecond solution which is a very dilute solution of the surfactant inacetone. Since PVP is not soluble in acetone, the intermediate layerconsisting of this compound is not attacked in this second coatingoperation. In the case of an intermediate layer formed from thebasic-soluble Bellac, by contrast, a dilute aqueous surfactant solutionwith a pH not exceeding 9.5 is used. This does not attack the Bellaclayer. On account of the highly dilute nature of the surfactantsolution, only a monomolecular surfactant layer is formed on theintermediate layer of PVP or Bellac, and the nonpolar groups of thissurfactant layer are uniformally facing outward, thereby producingnonpolar particles with little tendency to agglomerate and good flowproperties.

To produce an object from particles of this type, a layer of theseparticles is placed on a base and sprayed from above with a binderliquid in accordance with a predetermined pattern. For this sprayingoperation, it is possible to use an appliance similar to a generallyknown ink-jet printer; appliances of this type are described in theEuropean patents referred to in the introduction and are not explainedin more detail here.

Suitable binder liquids are basic aqueous solutions, such as for exampleammonia solution, which were also used for depositing the intermediatelayer of Bellac, since they dissolve both intermediate layers and thesurfactant layer but not the PMMA core. To set a desired viscosity ofthe binder liquid, it is possible, for example, to add glycol.

Printing parts of the particle layer with the binder liquid at leastpartially dissolves the surface layer and the intermediate layer but notthe core which they enclose. The binder liquid is evaporated/vaporized,and the joined-together coatings together with the cores which theyenclose remain behind, so that the previously separate particles form acohesive body; cf. in this respect the joined, hatched particles inFIG. 1. In the area surrounding this region, which has not been affectedby the binder liquid, the particles remain separate, in unchanged form.

Repeated application of a layer of particles to the previous layer andprinting of regions of the new layer with binder liquid in accordancewith a predetermined pattern, which may vary from layer to layer,ultimately produces a cohesive body formed from joined particles, fromwhich it is then merely necessary to remove the surrounding particleswhich have remained separate.

Since the basic aqueous ammonia solution used as binder liquid does notdissolve the PMMA cores of the particles, their original form remainsunchanged in the finished object, and consequently the shrinkage of thefinished object can be no stronger than the ratio of the thickness ofthe intermediate layer to a mean radius of the cores of the particles.This thickness may, for example, amount to 0.5 μm for a mean radius ofapprox. 10 μm.

The nonpolar nature of the outer surfaces of the particles makes thepowder insensitive to atmospheric humidity and thereby preventsagglomeration of the particles. It therefore ensures the formation ofuniform spaces between the unjoined particles and accordingly alsoensures a uniform propagation of binder liquid which is sprayed on.Consequently, the surfaces of the object obtained are uniformally smoothand accurately follow the predetermined pattern of the distribution ofthe binder liquid.

In an exemplary embodiment for 3D laser sintering, the particles have acore of polymethyl methacrylate (PMMA), which is surrounded by anintermediate layer with a low softening point (<70° C.), in this case apoly(ethylene glycol)amine or —amide. This material forms a polar,hydrophilic outer surface. Suitable poly(ethylene glycol)amines or—amides are listed in known databases, such as BEILSTEIN or GMELIN.

The intermediate layer is produced by dissolving the poly(ethyleneglycol)amine in methanol, applying the solution to the particles anddrying the particles. Other suitable solvents are tertiary butyl methylethers or ethyl acetate, each likewise with low boiling points. For thecoating operation, the particles are fluidized in a fluidized bed by anairstream and are simultaneously sprayed with the solution. Drops of thesolution which come into contact with particles and surround themevaporate in the airstream, with the dissolved poly(ethyleneglycol)amine remaining behind and forming the intermediate layer. Theresulting layer thickness can be controlled using the concentration ofthe solution used and the duration of the treatment.

The surface layer formed from the surfactant sodium lauryl sulfate isproduced in a similar way to the intermediate layer by spraying theparticles which have been fluidized in the fluidized bed with a secondsolution, which is a highly dilute solution of the surfactant in arelatively long-chain alcohol, e.g. ethanol or propanol. Sincepoly(ethylene glycol)amine is not soluble in relatively long-chainalcohols, the intermediate layer consisting of this material is notattacked in this second coating operation. On account of the highlydilute nature of the surfactant solution, after evaporation of thesolvent only a monomolecular surfactant layer is formed on theintermediate layer, and the nonpolar groups of this surfactant layer areuniformally directed outward, thereby producing nonpolar particles withlittle tendency to agglomerate and good flow properties.

To produce an object from particles of this type, a layer of theseparticles is placed on a base and irradiated from above with a laserbeam in, accordance with a predetermined pattern, corresponding to across-section of the object, so that the particles are joined in theselected part. Then, the application and irradiation steps are repeatedfor a plurality of layers, so that the joined parts of the adjacentlayers are joined together in order to form the object. A process ofthis type and a suitable apparatus are known, for example, from DE 69031 061 T2 and are not explained in more detail at this point.

Since the softening point of the PMMA cores of the particles, at approx.124° C., is significantly higher than that of poly(ethyleneglycol)amine, at approx. 60° C., the original form of these cores isretained unchanged in the finished object if the introduction of laserenergy is suitably restricted, with the result that the shrinkage of thefinished object can be scarcely any greater than the ratio of thethickness of the intermediate layer to a mean radius of the cores of theparticles. This thickness may, for example, be 0.5 μm for a mean radiusof approx. 10 μm. The softening point of the surfactant monolayer is ofno importance on account of its low thickness and the associated minimalabsorption of energy.

The nonpolar nature of the outer surfaces of the particles preventsagglomeration of the particles before the softening of the (surface and)intermediate layer and thereby ensures uniform spaces between theunjoined particles and accordingly also ensures a uniform introductionof energy from the laser beam into the layer of particles. Consequently,the surfaces of the object obtained are uniformally smooth andaccurately follow the predetermined pattern of the laser irradiation.

1. A particle for producing a three-dimensional object by means of layer-building processes, comprising a core of at least a first material, a first coating on the core with a second material, which is polar, and a second coating on thc first coating, wherein the thickness of the first coating corresponds to 0.1 to 10% of the mean particle radius, wherein the second coating is formed from surfactant, the thickness of which corresponds to a monolayer of the surfactant, and wherein a uniformly non-polar outer surface of the particle is formed.
 2. The particle as claimed in claim 1, wherein the first coating and the second coating are soluble in water or an aqueous solution but the core is not.
 3. A process for producing a three-dimensional object, including the following steps: applying a layer of particles according to claim 1 to a target surface, irradiating a selected part of the layer, corresponding to a cross-section of the object, with an energy beam, so that the particles are joined in the selected part, repeating the application and irradiation steps for a plurality of layers, so that the joined parts of the adjacent layers are joined together in order to form the object.
 4. A process for producing a three-dimensional object, including the following steps: applying a layer of particles according to claim 1 to a target surface, printing a liquid in which at least parts of the particles are soluble onto a selected part of the layer, corresponding to a cross-section of the object, so that the particles are joined in the selected part, repeating the application and printing steps to form a plurality of layers, so that the joined parts of the adjacent layers are joined together in order to form the object. 